Daesh attacked Syrian regime forces and their allies in the countryside east of Homs and Hama on Wednesday, the Syrian Observatory for Human Rights (SOHR) said.
Heavy fighting raged around the mountainous Jabal al Shomariya area in Homs province, where the regime was carrying out air strikes, the British-based monitoring group said.
Pro-regime media outlets quoted a military source saying warplanes had hit targets in the eastern Hama countryside, which borders Jabal al Shomariya to the north.
Daesh has been losing ground to government forces further east, close to its stronghold of Deir Al Zor province and Al Sukhna, the last town it holds in Homs province.
Several sides in Syria's crowded battlefield are fighting Daesh, including regime forces backed by Russia and Iran, and US-backed, SDF around its Raqqa stronghold.
The SDF is dominated by YPG, which is the armed wing of the PYD. The PYD is the main partner for the US-led coalition against Daesh in Syria. Ankara considers the PYD to be the Syrian branch of the PKK, which is recognised as a terrorist group by Turkey, the US and EU.
Russian losses in Syria
Ten Russian servicemen have been killed fighting in Syria so far this year, according to statements from the Defence Ministry.
But Reuters estimates that at least 40 Russian servicemen and private military contractors have been killed in Syria so far this year, four times the official death toll.
The data may be on the conservative side, as commanders encourage the families of those killed to keep quiet, relatives and friends of several fallen soldiers, both servicemen and contractors, said on condition of anonymity.
Russia's Defence Ministry denied a Reuters report about a rise in the losses suffered by Russia in its military campaign in Syria, calling it "a lie from beginning to end," Russian news agencies reported.
"This is not the first time that Reuters is attempting to discredit by any means Russia's operation aiming to destroy Islamic State [Daesh] terrorists and return peace to Syria," the agencies cited Defence Ministry spokesman Igor Konashenkov as saying.
Kremlin spokesman Dmitry Peskov said on Wednesday that any Russian private citizens fighting with pro-regime forces in Syria are volunteers and the ministry has nothing to do with them.
The true level of casualties in the Syrian conflict is a sensitive subject in a country where positive coverage of the conflict features prominently in the media and ahead of a presidential election next year that incumbent Vladimir Putin is expected to win.
The scale of Russian military casualties in peace time has been a state secret since Putin issued a decree three months before Russia launched its operation in Syria. While Russia does not give total casualties, it does disclose some deaths.